I assume you are referring to metrology stardards such as surface plates, slip gauges etc?
The normal sequence is AA, A, B and Workshop with AA being better. I have seen references to AAAA and AAA as well.
In everyday parlance engineers refer to Inspection grade and Workshop grade. I expect there will be a BS or ISO specification defining exactly what the various grades mean.
In practise, critical inspection will be dom
ne by laser probing these days so tools to these standards will become rarer with just a few in glass cabinets to calibrate the lasers.
AA would be the best and kept for calibration, A would be for inspection and B is workshop grade. there is also a similar number system depending what the actual tool is 0 being calibration, 1 inspection and 2 workshop.
I think the letters are for things made to BS specs and the numbers to DIN spec
Seen a cabinet full of new engineers squares (mainly by Groz) all fairly cheap, but claiming AA grade. I assumed AA was better than A but thought I'd check.
Actually I was just after a small square for setting up fences on my jointer, checking drill bits in my drill press etc and keeping at hand for some layout. Nothing fancy, but then I see all these grades and it throws all sorts on confusion into the pot!
Get any new square, the grade isn't really that key. If you really insist on buying a calibrated one, don't handle it without gloves on, keep it in a controlled humidity and temperature environment at all times etc... in short, it's not worth it. Groz are eastern european I think and pretty decent.